1. Field of the Invention
The present invention relates to a recording apparatus employing an electrostatic latent image retaining member.
2. Description of the Prior Art
First of all, a recording apparatus employing an electrostatic latent image retaining member will be briefly described hereinunder.
A reproducing apparatus, which effects reproduction by exposing a document through scanning, consists of the following description: when a document to be reproduced is mounted on a document glass plate and a reproducing button is depressed, while irradiating the document an exposure lamp travels in a predetermined direction, together with an optical system including a reflecting mirror and other optical members. The reflected ray of light, corresponding to the shade of the document, is applied through the optical system to an electrostatic latent image retaining member (e.g., a photosensitive drum) which is uniformly charged, thereby allowing an electrostatic latent image to be formed on the electrostatic latent image retaining member. Moreover, a toner image corresponding to the shade of the document is formed on the electrostatic latent image retaining member by means of a developer.
On the other hand, a recording medium (e.g., a reproducing sheet of paper) is fed from a feeder so as to be coincident with the toner image position on the electrostatic latent image retaining member and comes in contact with the electrostatic latent image retaining member. Then, the toner image formed on the surface of the electrostatic latent image retaining member is transferred to the reproducing sheet by means of a transfer electrode. In the meantime, the electrostatic latent image retaining member continues rotating in a predetermined direction, and the toner image is gradually transferred to the reproducing sheet. Thereafter, the reproducing sheet having the toner image transferred thereto is separated from the electrostatic latent image retaining member and fed to a fixing device. This fixing device, e.g., a roller fixing device which is constituted by two rollers, at least one of which is heated, fixes the image formed by the developer to the reproducing sheet on heating. Thereafter, the reproducing sheet is delivered to the outside of the recording apparatus body.
A developing device, in the recording apparatus constructed as above, which develops the latent image formed on the electrostatic latent image retaining member into a visible image by means of a developer will be explained hereinunder in more detail.
The electrostatic latent image retaining member is uniformly charged and then exposed correspondingly to the reflected ray image. Consequently, the electrostatic latent image retaining member has on its surface, portions retaining electrostatic charges and portions retaining no electrostatic charges, thereby to form a latent image. When a developer comes in contact with the electrostatic charge retaining portions, the developer (a toner in the case of a two-component developer; and a one-component toner in the case of a one-component developer) is attracted by means of an electrostatic force between the charges of the latent image and the charges of the developer to develop a visible image formed by the developer (toner) on the electrostatic latent image retaining member. The visible image (toner image) is transferred to a reproducing sheet in the subsequent step.
The toner is consumed by adhering to the charged portions of the electrostatic latent image retaining member, that is, through development and therefore must be refilled according to the number of reproductions.
As a developing means to attain the above object, a magnetic brush developing device is generally known and frequently employed.
FIG. 1 schematically shows a magnetic brush developing device.
The following description is in the case of a one-component magnetic developer.
As a magnetic developer 14 (the developer is employed to mean a one-component magnetic toner in the following description) is carried on the peripheral surface of a developer carrier 13 incorporating therein a rotatable magnet 16 and the magnet 16 is rotated in the direction of the illustrated arrow, the developer 14 is carried in the direction opposite to the direction of rotation of the magnet 16 along the peripheral surface of the developer carrier 13. If an electrostatic latent image retaining member 9 is provided in close proximity to this developer carrier 13, the developer 14 carried along the peripheral surface of the developer carrier 13 comes in contact with the electrostatic latent image retaining member 9 at a developing area C. When the electrostatic force between the charges of the charged portions (electrostatic charge retaining portions) of the electrostatic latent image retaining member 9 and the charges of the developer 14 is larger than the magnetic force of the developer carrier 13 which attracts the developer 14, the developer 14 is transferred from the developer carrier 13 to the electrostatic latent image retaining member 9 to adhere thereto, thereby to form a visible image on the electrostatic latent image retaining member 9, which latent image is transferred to another member, such as a reproducing sheet or the like, in the subsequent step.
If the electrostatic latent image retaining member 9 having a latent image formed thereon and the developer 14 do not sufficiently come in contact with each other, the developer 14 will not adhere to the whole surface of the charged portions of the electrostatic latent image retaining member 9, resulting in the formation of a coarse visible image on the electrostatic latent image retaining member 9. Such an undesirable phenomenon occurs, as a matter of course, due to lack of the developer 14 on the developer carrier 13. In the conventional magnetic brush developing device, however, the above-mentioned phenomenon may occur even when there is a sufficient amount of the developer 14 on the developer carrier 13.
As the result of a close observation of the magnetic brush developing device in its developing state, the present inventors have found out that a visible image is formed on the electrostatic latent image retaining member 9 in such a process as mentioned hereinafter.
As shown in FIG. 2, the developer 14 is carried along the peripheral surface of the developer carrier 13 in the direction opposite to the direction of rotation of the magnet 16 in the state where a thick layer of the developer 14 is formed above each of magnetic poles A of the magnet 16, while a thin layer of the developer 14 is formed above each of areas B between the adjacent magnetic poles.
In addition, as shown in FIG. 3, only when inside magnetic pole area A surrounding inside the developer carrier 13 passes developing area C, and the layer of the developer 14 is sufficiently thick, will the electrostatic latent image retaining member 9 and the developer 14 come in contact with each other (wherein, the contact is effected through only a short width l). However, as shown in FIG. 4, when each area B between the adjacent magnetic poles passes developing area C, the layer of the developer 14 is thin and, therefore, no developer 14 is substantially in contact with the electrostatic latent image retaining member 9. Moreover, even when the magnetic poles of magnetic 16 face the electrostatic latent image retaining member 9, the magnetic lines of force produced from one magnetic pole which extend toward the electrostatic latent image retaining member 9 are smaller in number than those toward the adjacent opposite magnetic poles. The degree of concentration of the developer 14 is substantially proportional to the magnetic flux density of the magnetic pole. Therefore, even if one magnetic pole N passes to the developing area C as shown in FIG. 3, when the magnetic flux density is low, the developer 14 is not sufficiently concentrated on the part above the magnetic pole N, resulting in a reduction in the width l of the area of contact between the electrostatic latent image retaining member 9 and the developer 14. Accordingly, even if a charged portion of the electrostatic latent image retaining member 9 passes the developing area C, the developer 14 is not sufficiently attracted, so that the visible image formed by the developer 14 becomes coarse, undesirably.
The present inventors have devised a recording apparatus which makes it possible to increase the magnetic flux density on the electrostatic latent image retaining member at the developing area to obtain a high-density recording, even when the conventional magnetic brush developing device, for example, is employed to form a visible image on the electrostatic latent image retaining member.